Coins have been used as a form of currency across the world for almost three thousand years. Much like today, the earliest coins were formed from metals, with more precious metals reflecting higher monetary value. The most common shape of coin is a flat, rounded disc shape, which also tends to be small in size. Antique and rare coins can hold high value on the collectors' market, and coins produced to commemorate a specific event can be of particular interest to numismatists. In the UK, coins have been used to mark new royal marriages, with the commemoration of Prince Charles and Lady Diana's wedding in 1981 perhaps being the most recognizable example from the British Royal Mint. Between 1975-6, the United States produced a series of Bicentennial coins, commemorating 200 years since the Declaration of Independence.